1. Field of the Invention
The present invention relates to an electric power supply system for supplying energy to a vehicle from outside of it by a non-contact method.
2. Description of the Related Art
As an increasing number of people have been paying more and more attention to environmental issues these days, there have been growing expectations for electric vehicles to be driven with batteries and electric motors. Such an electric vehicle can naturally be charged and powered via cable connection. But people have been researching and developing wireless charging and powering techniques that adopt a non-contact method. Meanwhile, considering the overall weight and cost of batteries, there is definitely a limit to the overall capacity of batteries that can be loaded into a single vehicle. And that limit to the overall capacity of batteries directly leads to the maximum mileage a vehicle can do per charging. That is why there is an increasing demand for developing an effective technique for charging and powering a running vehicle.
As a wireless power transmission technique, a resonant magnetic coupling method was proposed in United States Patent Application Publication No. 2008/0278264-A1 (see its description and FIGS. 6 and 11, among other things). According to such a method, resonant mode coupling between resonator antennas is used, and therefore, power can be transmitted more efficiently over a longer distance than conventional electromagnetic induction methods. Among other things, since a resonant magnetic field is used according to such a method, influence on surrounding organisms would be much less than a situation where a resonant electric field is used.
The wireless power transmission technique by resonant magnetic coupling can increase the transmission distance by leaps and bounds compared to the conventional electromagnetic induction method. Specifically, if the coupling coefficient k indicating the degree of coupling between resonant antennas is greater than the square root of the product of the attenuation constants Γ1 and Γ2 of the respective antennas, energy can be transferred efficiently. A wireless power transmission system that uses the resonant magnetic coupling uses a resonant circuit that has a coupling coefficient k of less than one but has a high Q factor indicating that the resonance will produce low loss, and therefore, can be regarded as an electromagnetic induction system with increased transmission efficiency. However, since the resonant magnetic coupling method requires that k≠1 unlike the electromagnetic induction method in which k≈1, power cannot be transmitted with high efficiency unless inter-block impedance matching is kept achieved between the oscillation section and the power transmitting antenna and between the rectification section and the power receiving antenna.
Japanese Patent Application Laid-Open Publication No. 2009-106136 discloses an example in which the technique of United States Patent Application Publication No. 2008/0278264-A1 is used to charge and power a vehicle. According to this patent document, while a parked vehicle is being charged and powered, the relative gap between a power transmitting antenna, which is installed on the ground, and a power receiving antenna, which is arranged on or in the vehicle, can be filled by arranging multiple sets of power transmitting and receiving antennas.
On the other hand, United States Patent Application Publication No. 2008/0265684-A1 discloses a system for transferring power to a vehicle by electromagnetic induction. This patent document teaches how to cope with horizontal parking position shift of a vehicle being parked and charged. That system provides two power receiving antennas and two power transmitting antennas and controls their amplitudes and phases independently of each other, thereby dealing with such a positional shift.
In both of the system for charging and powering a running vehicle and the system for charging and powering a parked vehicle, the stability of transmission could be lost due to a relative positional shift between the power transmitting and receiving antennas. The present inventors point out this problem and provide means for overcoming it.
Specifically, in the system for charging and powering a vehicle by resonant magnetic coupling, if the relative position of the power receiving antenna in a running or parked vehicle to the power transmitting antenna shifts horizontally with respect to the traveling direction of the vehicle, the power transmission by resonant magnetic coupling would lose its stability.
Particularly, as far as running vehicles are concerned, most vehicles will run along approximately the center of a traffic lane but quite a few vehicles will also run along either end of the traffic lane. That is why the relative horizontal positional shift of a running vehicle would be greater that of a parked vehicle. United States Patent Application Publication No. 2008/0265684-A1 provides means for coping with the loss of stability of the transmission performance due to relative horizontal positional shifts between the power transmitting antenna and the power receiving antenna of a parked vehicle with respect to the traveling direction of the vehicle. According to the technique disclosed in that patent document, however, the number of antennas to provide should be increased, thus raising the cost significantly. On top of that, since charging and powering should not be done efficiently that way, it is difficult to apply such a technique to a system for charging and powering a running vehicle.
Likewise, even in a system for charging and powering a parked vehicle, there will also be relative horizontal positional shifts with respect to the running direction of the vehicle. If the driver should have such high driving skills as to park his or her vehicle exactly where it is supposed to be, then there would be no horizontal shift at all. However, if such an unrealistic restriction were imposed, the advantage to be achieved by adopting such a non-contact charging and powering system would be sacrificed. For that reason, even for the system for charging and powering a parked vehicle, a scheme for ensuring stabilized performance in cases of horizontal shifts should also be provided. Nevertheless, Japanese Patent Application Laid-Open Publication No. 2009-106136 does not mention what problems will arise when a system for charging and powering a running or parked vehicle is established and provides no means for overcoming such problems.
It is therefore an object of the present invention to provide an electric power supply system for vehicle that can charge and power a running or parked vehicle while achieving stabilized performance even when the vehicle shifts horizontally with respect to its traveling direction.